Foam forming has a long history. For example, techniques of obtaining resin foamed products by injection foaming are disclosed in U.S. Pat. Nos. 3,268,639 and 3,384,691. In recent years, methods of foam forming using chemical foaming agents or physical foaming agents are described in synthetic resin foaming textbooks.
Recently, a method and an apparatus for forming a very small foam called a microcell was described by the Massachusetts Institute of Technology, U.S.A., in U.S. Pat. Nos. 4,473,665, 5,158,986, 5,160,674, 5,334,356, 5,571,848, and 5,866,053. In the method and apparatus proposed by the Massachusetts Institute of Technology, U.S.A., a supercritical inert gas is blown into a portion where a resin is melted in a plasticator of an injection foaming machine, and the well-melted resin and the gas are mixed by a static mixer. It was reported that when the pressure and temperature were controlled, a large number of cells of 25 μm or less were evenly dispersed in the formed foamed product, and the foamed product had almost no strength deterioration since the cell size was small. The above references also describe a method by which a resin material is placed in a pressure vessel, a supercritical inert gas is allowed to saturate into the resin material, and the resin is foamed by abruptly reducing the pressure at a temperature near the melting temperature of the resin, and a method by which the resin is foamed by abruptly raising the temperature after the temperature and pressure are once lowered.
Japanese Patent Laid-Open Nos. 8-85128 and 8-85129 describe methods by which a pressure-resistant chamber is attached to a hopper of an injection foaming machine, and a gas is allowed to saturate into a molten resin or resin pellets at a high pressure, thereby foaming the resin.
That is, the conventional foam forming is roughly classified as chemical foaming and physical foaming. The chemical foaming includes a master batch method in which a foaming agent which causes a chemical reaction by heat and a resin material are mixed in the form of pellets, and a method in which the foaming agent is kneaded in a resin material. The physical foaming includes a method in which an inert gas is allowed to directly saturate into a molten resin from a plasticator of an injection foaming machine or extruder, and a batch method in which an inert gas is allowed to saturate into a previously foamed and shaped resin product at a high temperature and high pressure, and the resultant material is foamed in a pressure vessel by abruptly changing the temperature or pressure.
In the conventional chemical foaming, the master batch method by which a heat decomposable foaming material and a resin material are mixed immediately before foaming is often used. Unfortunately, this method has many problems such as harmfulness, mold corrosion, the worsening of the foaming environment, and the difficultly of handling. In contrast, the physical foaming is harmless and causes no mold corrosion, and nitrogen and carbon dioxide exist in natural air. Therefore, the physical foaming is regarded as superior to the chemical foaming. However, in the method of allowing an inert gas to saturate directly into a molten resin, the gas is directly blown into the molten resin material, so a portion of the molten resin in contact with the gas is rapidly cooled when the gas is blown. If the gas is continuously blown, a large portion of the molten resin is cooled. Consequently, the viscosity rises, and it takes a long time to restore the resin temperature and viscosity suited to foaming.
Also, when a gas is heated to a temperature close to the melting temperature of a resin in advance, the volume of the gas increases with the temperature rise. Therefore, if the gas is directly blown into the molten resin, the foaming magnification after the resin is charged into a foam significantly decreases because the internal pressure of the resin is low.
Furthermore, to compensate for this drawback, it is possible to raise the pressure together with the gas temperature, and blow the gas into the molten resin while the gas concentration is maintained. In this method, however, the gas pressure is very high, and the gas flows into the molten resin at the moment when the gas is blown into it. This makes it difficult to control the gas blowing amount, and increases variations in the amount of the gas, which saturates into the resin. Also, since the gas is abruptly blown into the molten resin, the molten resin forms two separated layers of the gas and resin near the blowing port. Therefore, to evenly disperse the gas in the resin, it is necessary to mechanically repeat the kneading by using a static mixer or the like, and encourage dissolution of the gas into the resin by raising the pressure of the resin-gas mixture itself. This complicates the apparatus, and variations in gas saturation amount in the material vary the dimensional accuracy of the foamed product and deteriorate the product quality. In addition, the long cycle reduces the productivity.
The batch method using an inert gas eliminates both the drawbacks of the chemical foaming and the drawbacks of the physical foaming by which a gas is directly blown into a molten resin. However, since this batch method is an intermittent production method using batch processing, the productivity is significantly reduced.
A method such as the one described in Japanese Patent Laid-Open No. 8-85128, in which a gas is allowed to saturate into a resin material by the batch method and then the material is continuously foamed by an injection foaming machine, compensates for the above-mentioned drawbacks. However, the amount of the gas which saturates into a solid resin material, such as pellets, changes in accordance with the gas pressure, temperature, and time. Accordingly, if the foaming cycle or gas saturation time changes even slightly, the amount of the gas which saturates into a resin material changes. Since this changes the foamed state of the foamed product, the accuracy also changes.